Need to forward a custom port to an existing port on your Linux box?
Simply use the following command as root :
iptables -t nat -I PREROUTING --src 0/0 --dst 0.0.0.0/0 -p tcp --dport <port-to-forward> -j REDIRECT --to-ports <destination-port>
Example : Assuming you want to forward port 1025 to port 587 :
iptables -t nat -I PREROUTING --src 0/0 --dst 0.0.0.0/0 -p tcp --dport 1025 -j REDIRECT --to-ports 587
The command above won’t survive to reboot, see the configuration below to enter in the IPTables script :
-A FORWARD -d 0.0.0.0/0 -p tcp -m tcp --dport 1025 -m comment --comment "Forward inbound connections from tcp1025 to tcp587" -j ACCEPT